1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a facsimile apparatus or a printer, and more particularly, to an electrophotographic image forming apparatus having an LED (Light Emitting Diode) print head capable of exposing the surface of the photoreceptor in a plurality of gradations.
2. Description of the Prior Art
Generally, in image forming apparatuses, light of a quantity corresponding to the gradation of the inputted image data is applied from the LED print head to the photoreceptor, and by doing this, the surface of the photoreceptor is exposed to thereby form an electrostatic latent image of the input image. The LED print head is opposed to the surface of the drum-shaped photoreceptor with a predetermined distance in between, and extends in the axial direction of the photoreceptor. In the LED print head, for example, approximately 7000 LEDs are linearly arranged in accordance with the resolution.
The lighting time of the LEDs of the LED print head is changed so as to correspond to the gradation of the image data by being controlled by a preset gradation pulse of a lighting clock. For example, for high gradations, the lighting time is increased so that the exposure amount of the photoreceptor surface that is substantially proportional to the lighting time is increased, whereas for low gradations, the lighting time is decreased so that the exposure amount of the photoreceptor surface is decreased. Consequently, for the former high gradations, the amount of toner adhering to the exposed area increases, so that an image with high density is developed, whereas for the latter low gradations, the amount of toner adhering to the exposed area decreases, so that a light image is developed.
In the above-mentioned conventional lighting clock, the increment of the gradation pulse between the gradations is fixed so that the increment of the lighting time between the gradations is fixed. However, the relationship between the lighting time of the LEDs, that is, the exposure amount of the photoreceptor surface and the amount of toner adhering to the exposed area (the toner image density (ID)) is not uniformly linear, but there are nonlinear parts in the initial and closing stages (see FIG. 10). Therefore, even if the lighting time is linearly increased, that is, is made linear in light quantity in proportion to the gradation of the input image data, the relationship between the gradation of the input image data and the density of the formed toner image is not linear. In particular, for low gradations and high gradations, the density of the toner image and the density of the input image data are subtly different.
To solve such a problem, as a conventional improved technology, an image forming apparatus comprising the following has been proposed: setting means for nonlinearly setting an appropriate lighting time appropriate for each gradation so that the increment of the toner image density between the gradations is fixed based on the correlation between the toner image density and the lighting time, forming a lighting clock having a gradation pulse coinciding with a multiple of a reference pulse of its own reference clock with substantially the same timing as the appropriate lighting times, and setting the formed lighting clock; and LED driving means for counting the number of gradation pulses by receiving the lighting clock set by the setting means in accordance with the gradation of the inputted image data and lighting the LEDs until each number of gradations is reached (for example, see Japanese Laid-Open Patent Application No. 2002-248808 [pages 2–7, FIGS. 8 and 9]). According to such an image forming apparatus of the improved technology, LEDs are lit under a condition where for low gradations and high gradations, the lighting time, that is, the increment of the gradation pulse is set to a high value and for intermediate gradations, it is set to a value lower than that in order that the toner image density substantially corresponds to the gradation of the input image data, so that the density of the toner image and the density of the input image data are the same over the entire gradation range. Thus, the above-mentioned problem is tolerably solved.
In the sensitivity characteristic of the photoreceptor surface, there are variations of a certain extent among individual products, and in the light quantity characteristic of the LEDs of the LED print head, there are also variations by approximately (the highest value/the lowest value at the light quantity of the same lighting time)>2 among individual products. Consequently, the exposure performance varies among image forming apparatuses according to the combination of the photoreceptor and the LED print head provided in the image forming apparatus. According to the conventional lighting clock, since the increment of the gradation pulse between the gradations is fixed, the variation amount cannot be permitted, so that the toner image density gradually significantly varies from intermediate to high gradations among image forming apparatuses.
Moreover, LEDs cannot always emit a stable quantity of light immediately after the start of lighting, and in actuality, a loss time during which no exposure is performed is present immediately after lighting. Consequently, unless the lighting time (gradation pulse) of the LEDs is set with consideration given the loss time, for low gradations, particularly for the first gradation, the lighting time is substantially short and the desired exposure amount cannot be obtained, so that an image commensurate with the gradation is not formed.
On the other hand, in the case of the above-described conventional improved technology, since the lighting clock is formed from the reference clock, gradation pulses of the lighting clock that are suitable for each image forming apparatus can be freely set somehow or other. However, since the sensitivity characteristic of the photoreceptor surface is not sufficiently considered in setting the gradation pulses, it cannot be said that variations in exposure performance among image forming apparatuses can be handled.
In this improved technology, examining the relationship between variations in the sensitivity characteristic of the photoreceptor and the light quantity characteristic of the LEDs and appropriate lighting times T1/16, T2/16, . . . , and T15/16 of the gradations, when the photoreceptor sensitivity characteristic and the LED light quantity characteristic are both the lowest values, as shown in FIG. 11, a sufficient increment width of the gradation pulse can be secured even in the case of intermediate gradations where the increment of the appropriate lighting time is small. However, when the photoreceptor sensitivity characteristic and the LED light quantity characteristic are both the highest values, as shown in FIG. 12, the increment width of the appropriate lighting time in intermediate gradations is extremely small, so that it is difficult to secure the increment width of the gradation pulse. This is because although the increment width of the gradation pulse coincides with a multiple of the reference pulse of the reference clock with substantially the same timing as the increment width of the appropriate lighting time, in actuality, it is restricted by the minimum width that the reference pulse can have. Thus, this improved technology leaves a problem in practicality. Needless to say, it is difficult to reflect the above-mentioned loss time.